A solid oxide fuel cell (hereinafter referred to as “SOFC”) is highly efficient and does not require an expensive catalyst such as platinum. On the other hand, since its operating temperature is as high as 800° C. to 1000° C., a problem arises in that a structural material such as an interconnector is likely to be degraded.
To solve the above-described problem, an intermediate temperature operating SOFC having a lowered operating temperature less than or equal to 600° C. has been expected. At low operating temperatures, however, efficiency is decreased, so that predetermined power generation performance cannot be ensured disadvantageously. Therefore, a solid electrolyte exhibiting high efficiency even at low operating temperatures and being capable of ensuring predetermined power generation performance has been required.
As a solid electrolyte, one having oxygen ion conductivity or proton conductivity is employed. In the case of employing a solid electrolyte having oxygen ion conductivity, the oxygen ion is bonded to hydrogen to produce water at a fuel electrode. This water dilutes fuel to decrease efficiency disadvantageously.
On the other hand, a solid electrolyte having proton conductivity such as yttrium-doped barium zirconate (hereinafter referred to as “BZY”) can achieve high proton conductivity at low temperatures since activation energy for charge transfer is low, and is expected as a solid electrolyte material as an alternative to the above-described solid electrolyte having oxygen ion conductivity. In the case of employing the solid electrolyte having proton conductivity, the above-described problem encountered in the solid electrolyte having oxygen ion conductivity does not occur.